Traditionally, vitreous materials have been characterized as being brittle and having a tendency to shatter into sharp pieces or shards on breakage. Consequently, these materials have not been amenable to machining processes, as are metals, but have been molded either in a thermally softened state or as a comminuted powder.
Approximately twenty years ago, glass-ceramics, that is, ceramic-like materials produced by controlled crystallization of certain glasses, were announced. Like the glasses that they are produced from, these new glass-ceramic materials tend to be brittle and to shatter when subjected to normal machining processes.
It was surprising indeed when it was discovered recently that certain glass-ceramics were capable of being machined. While these machinable materials have achieved technical success, and are used in many special applications, their cost is a limiting factor on broader acceptance. It would, then, be particularly desirable to provide machinable glasses and/or glass-ceramics at a lower cost, and this is a basic purpose of the present invention.